1. Field of the Invention
This invention relates to an electric motor and, more particularly, to such an electric motor in which speed detecting magnets are provided outside the bottom of a rotor casing and a speed detecting winding is provided concentrically with the detecting magnets and wherein the driving current of the motor is controlled by a voltage induced in the detector winding in response to the rotation of the detector magnets.
2. Description of the Prior Art
Tape drive capstans of tape recorders are generally driven by electric motors which ideally have a minimum fluctuation in rotational speed with a maximum service life. It has been the recent trend to employ brushless motors which do not have a commutator and brush for such purposes. However, in the conventional brushless motor, a motor torque generating section consisting of stator windings, rotor magnets and a detector element for detecting the rotational angular position of the rotor magnets is provided separately from a control circuit section which controls current supply to the stator windings in response to the detecting signals from the detector element. Such brushless motors also have a considerable number of lead wires for connecting the two sections. Thus it is tedious to assemble and connect such brushless motors and the space requirements necessary to accommodate such motors becomes prohibitive. It is also a problem in that the manipulation of such a motor is cumbersome.
There are also additional disadvantages in that, where such a conventional brushless motor is used as a servomotor, it is the practice to utilize the voltage induced in the driving windings by the rotation of the driving magnets of the rotor as the signals corresponding to the speed of rotation. This results in the fact that the induced voltage includes substantial ripple components due to fluctuation of driving current when the number of poles is small. The ripple components must be eliminated by the use of other means, thereby requiring a complex circuit structure. Further, the magnitude of the output of the induced voltage tends to be affected by temperature variation so that it is often difficult to control the motor speed with a high degree of accuracy. Notwithstanding the presence of these disadvantages, it is essential for high quality performance to control with absolute accuracy the rotational speed of electric motors employed in audio devices such as tape recorders and record players.